1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray source, an X-ray detector, and an exposure control system for controlling the X-ray source and the X-ray detector, which exposure control system is arranged to control the X-ray source so as to carry out a test exposure with a low X-ray dose while producing a control signal by the X-ray detector, and to control the X-ray source on the basis of the control signal in order to carry out an X-ray exposure with a high X-ray dose and to acquire an X-ray image by the X-ray detector during this exposure.
2. Description of the Related Art
An X-ray examination apparatus known in the art is German Offenlegungsschrift DE 43 30 787.
An object to be examined during a radiological examination, for example a patient to be examined, is exposed to an X-ray dose in order to form one or more X-ray images of the object. The X-ray dose required for individual patients to be examined is customarily adjusted individually. The X-ray dose is adjusted by the exposure control system.
The known X-ray examination apparatus carries out the test exposure first with a comparatively low dose, thus forming a test image. During the formation of this test image, the patient to be examined is briefly exposed to X-rays with a predetermined intensity and energy. Because X-rays are applied only during a short period of time during the formation of the test image, the X-ray dose for forming the test image remains comparatively small. The test image is read out from the X-ray detector and digitized in digital grey scale values with, for example, a bit depth of 10 bits. The exposure control system of the known X-ray examination apparatus derives the X-ray dose which is required to form the X-ray image during the X-ray exposure from the distribution of the digital grey scale values. The X-ray dose required for the formation of the X-ray image is large in comparison with the X-ray dose required to form the test image. In the known X-ray examination apparatus, the larger X-ray dose is obtained by using a longer exposure time with the same intensity and energy of the X-rays as used during the test exposure. The known X-ray detector is provided with a sensor matrix having a large number of sensor elements.
It is a drawback of the known X-ray examination apparatus that the reading out of the X-ray detector after the test exposure requires the same amount of time as the reading out after the X-ray exposure whereby the X-ray image is formed. Consequently, the adjustment of the X-ray source for the X-ray exposure requires a comparatively long period of time.
It is an object of the invention to provide an X-ray examination apparatus enabling faster adjustment of the X-ray source.
This object is achieved by an X-ray examination apparatus according to the invention which is characterized in that the exposure control system is arranged to adjust the X-ray detector to a coarse spatial resolution during the execution of the test exposure, and to adjust the X-ray detector to a fine spatial resolution during the execution of the X-ray exposure.
As the X-ray detector is adjusted to a coarser resolution during the test exposure, fewer signal values need be read out from the X-ray detector. Consequently, the time required to read out the X-ray detector is shorter. Moreover, only a smaller number of signal values, for example brightness values, need be processed in order to derive the control signal therefrom; consequently, less time will also be required to form the control signal. Because the X-ray examination apparatus according to the invention requires only a short period of time for the adjustment of the X-ray examination apparatus on the basis of the test exposure, the X-ray image will become available sooner. More specifically, only a short period of time elapses between the test exposure and the X-ray exposure. Consequently, changes are less likely to occur between the test exposure and the X-ray exposure, for example due to motion in or of the object to be examined. The X-ray examination apparatus according to the invention is thus capable of quickly forming an X-ray image while ensuring that the X-ray image has a high diagnostic quality. This means that small details of low contrast are clearly reproduced nevertheless in the X-ray image. The adjustment of the X-ray examination apparatus concerns notably the adjustment of the energy and the intensity of the X-rays emitted by the X-ray source.
These and other aspects of the invention will be described in detail with reference to the following embodiments, limited only the scope of the claims appended hereto.
Preferably, the invention is used in an X-ray examination apparatus provided with an X-ray detector having a sensor matrix with a large number of sensor elements. The individual sensor elements convert incident X-rays into electric charges. The sensor matrix also includes read-out lines via which the electric charges formed in the sensor elements can be read out or detected. During the test exposure, electric charges are formed in the sensor elements. The separate signal levels of the control signal are composed of the electric charges in a comparatively small number of or in comparatively large groups of sensor elements. Such a large group contains a large number of sensor elements, i.e. more than one sensor element in any case. During the execution of the X-ray exposure, electric charges are formed again in the sensor elements. Any residual charges due to the test exposure are removed from the sensor elements. Such removal of electric charges is also called xe2x80x9celectric resetxe2x80x9d and is performed, for example by simultaneously reading out and draining the electric charges via all read-out lines. It is to be noted that the electric reset is known per se from U.S. Pat. No. 5,905,772 in which it is applied to counteract ghost images caused by electric charges left behind in the sensor matrix. After the X-ray exposure, the electric charges are read out again so as to form the image signal. The separate signal levels of the image signal are derived from a rather large number of or from comparatively small groups of sensor elements. Such a small group of sensor elements contains only a small number of sensor elements; for example, such a group contains only a single sensor element. The small groups of sensor elements contain fewer, preferably far fewer, sensor elements than the large groups of sensor elements. It is to be noted, however, that the sensor elements of the large groups as well as those of the small groups preferably form part of the whole assembly of sensor elements of the sensor matrix. Sensor elements may form part of a large as well as a small group. Furthermore, the sensor elements of the large as well as those of the small groups preferably have the same construction. For example, the sensor elements are constructed as photodiodes. The sensor matrix is provided with, for example, 1000xc3x971000 or even 4000xc3x974000 sensor elements. For example, a small group contains only a single sensor element or two neighboring sensor elements. For example, 4xc3x974 or 32xc3x9732 large groups of sensor elements are used for the test exposure. When electric charges are combined from large groups of sensor elements, less time will be required for the reading out of the sensor matrix. The spatial resolution of the X-ray detector is coarser as electric charges from more sensor elements are combined during reading out. The spatial resolution represents the dimensions of the smallest detail that is faithfully detected by the X-ray detector. It has been found that it is not necessary for the control signal that very small details are accurately detected. The advantage of the shorter period of time that is required for the reading out of the electric charges from the large groups outweighs the loss of spatial resolution. Moreover, the signal-to-noise ratio of the control signal is higher as larger groups of sensor elements are used, i.e. as the electric charges of more sensor elements are combined during the reading out.
For the reading out of the X-ray image formed by the X-ray exposure, however, the fine spatial resolution of the X-ray detector is of importance, to ensure that small details in the X-ray image are taken up exactly in the signal levels of the image signal. This fine spatial resolution is achieved by deriving the separate signal levels of the image signal from electric charges of respective small groups which preferably contain only one or two sensor elements.
Preferably, a test image is formed by the test exposure. A measuring field is selected from such a test image. The measuring field relates to a part of the object to be examined, for example the patient, which is very relevant for the adjustment of the X-ray source. The measuring field is determined, for example, on the basis of the brightness values of the test image. The control signal is derived from the brightness values of the measuring field. Because of the use of such a measuring field it is avoided notably that the control of the X-ray source is influenced by a part of the test image which does not relate to the part of the object to be examined. Notably, X-rays which reach the X-ray detector practically without attenuation are prevented from influencing the control of the X-ray source. The adverse effects of non-attenuated X-rays on the adjustment of the X-ray source during the X-ray exposure are thus avoided. For example, a histogram analysis can be performed on the brightness values of the test image, notably in order to prevent parts of the test image which relate to substantially non-attenuated X-rays from influencing the control signal. Underexposure of the imaging of relevant parts of the anatomy in the X-ray image is thus avoided. It is to be noted, however, that from U.S. Pat. No. 5,608,775 it is known to read out only a part of the columns of sensor elements during the test exposure.
Preferably, the dose rate is derived from the control signal produced by the test exposure. The dose rate represents the energy deposited on the X-ray detector by the X-rays per unit of time. The exposure control system is preferably arranged to calculate the time integral of the dose rate during the X-ray exposure. When the X-ray exposure is performed with the same intensity and energy of the X-rays as used for the test exposure, the time integral of the dose rate can be calculated simply by multiplying the dose rate found during the test exposure by the current duration of the X-ray exposure. It is alternatively possible to use an intensity and/or energy of the X-rays during the X-ray exposure other than that used during the test exposure. In order to calculate the time integral of the dose rate it is necessary to take into account the difference in dose rate during the X-ray exposure and during the test exposure. The dependency of the dose rate on the difference between the dose rate during the test exposure and the dose rate during the X-ray exposure can be calculated or determined by calibration measurements. The dependency of the dose rate on the difference between the dose rate during the test exposure and the dose rate during the X-ray exposure can be derived, for example, by performing two successive test exposures and measuring the dose rate at a different energy and/or intensity of the X-rays. The dose rate during the X-ray exposure can be calculated or accurately estimated on the basis of these two measuring results for the dose rate, for example by interpolation or extrapolation.
The time integral of the dose rate, so the current X-ray dose, is calculated, for example at regular intervals and compared with a preset X-ray dose. As soon as the current X-ray dose reaches or exceeds the preset value, the X-ray exposure is terminated by deactivation of the X-ray source. The X-ray source is thus adjusted to deliver the required, preset X-ray dose. For example, the signal level of the control signal represents the current X-ray dose. The exposure control system compares the signal level of the control signal with a limit value. Whenever the current X-ray dose is calculated again, the signal level of the control signal is adapted and the X-ray source is deactivated as soon as the signal level exceeds the limit level. The limit level amounts to, for example 1 V. In X-ray examination apparatus it is common practice to deactivate the X-ray source when the exposure control system supplies a control signal having a signal level exceeding the limit value of 1 V. The invention can thus be simply implemented in an existing X-ray examination apparatus.
The exposure control system in another embodiment of the X-ray examination apparatus according to the invention measures the mean X-ray intensity across a test field during the X-ray exposure. Such a test field comprises a pre-selected group or groups of sensor elements; it has been found that columns of sensor elements which are connected to the same read-out line are suitable to act as a test field. This is because it has been found that the total charge in the sensor elements of the test field can be detected within a short period of time. Such detection is possible notably by measurement across parasitic capacitances of the sensor elements and switching elements via which the sensor elements are connected to the read-out line. The total charge can thus be detected within 100 xcexcs; the duration of the X-ray exposure is typically from 2 to 10 ms or even 300 ms, so that the total charge can be detected twenty, one hundred or even thousands of times during the X-ray exposure. The mean intensity across the test field is thus updated during the X-ray exposure so that the current X-ray dose is known during the X-ray exposure. Individual read-out lines are connected to respective read-out amplifiers. It has been found that, due to parasitic signal transfer from the sensor elements connected (for example, per column) to the same read-out line, the output of the relevant read-out amplifier always presents a signal level which corresponds to the total charge in the sensor elements connected to the relevant read-out line. The total amount of the electric charges in the sensor elements connected to individual read-out lines can be measured at the output of the read-out amplifiers, irrespective of the control of the switching elements whereby the sensor elements are connected to the read-out lines. The sum of the electric charges in the sensor elements connected to the respective read-out line corresponds to the mean X-ray intensity incident across the sensor elements connected to respective read-out lines. Consequently, the mean X-ray intensity across the test field can be continuously measured during the X-ray exposure. When sensor elements are column-wise connected to respective read-out lines, the test field can be composed of one or more complete columns of sensor elements.
It is alternatively possible to use a plurality of read-out lines per column, each time groups of sensor elements of individual columns then being connected to individual read-out lines. Such groups of sensor elements then comprise each time parts of columns. It is then possible to compose the test field from a plurality of such groups of sensor elements. The shape and the size of the test fields can thus be selected in dependence on the architecture of the connection of the sensor elements to the read-out lines. Preferably, a reasonable correspondence exists between the test field and the measuring field; the mean X-ray intensity across the test field then yields, in conjunction with a small correction for the difference between the test field and the measuring field, the signal level for the control signal whereby the X-ray source is accurately controlled. In practice it often occurs that the test field comprises one or more complete columns of sensor elements. In that case often large differences exist between the test field and the measuring field and it is necessary to derive the control signal from the mean X-ray intensity across the test field and across the measuring field, while taking into account the difference between the test field and the measuring field. A suitable procedure for taking into account the difference between the measuring field and the test field is the determination of the ratio of the mean X-ray intensity across the measuring field to the mean X-ray intensity across the test field during the test exposure. This ratio is represented by a correction factor which amounts to the ratio of the mean X-ray intensity across the measuring field during the test exposure to the mean X-ray intensity across a test field during the test exposure. This ratio can be readily calculated on the basis of the brightnesses in the parts of the test image which correspond to the test field and the measuring field, respectively. The mean X-ray intensity across the measuring field can then also be derived during the X-ray exposure by multiplying the measured mean current X-ray intensity across the test field by the correction factor. Subsequently, time integration is applied to the current mean X-ray intensity thus calculated in order to calculate the current X-ray dose relating to the part of the patient to be examined.